1) Field of the Invention
The present invention relates generally to the field of second-generation superconductors. More specifically, it relates to a process for depositing, at a very high rate, a biaxially-textured film on a continuously moving metal substrate tape. Even more specifically, it relates to a process wherein a film is deposited on a substrate with a deposition flux having an oblique incident angle, while simultaneously being bombarded by an ion beam at an ion beam incident angle arranged along either a best ion texture direction (BITD) or a second best ion texture direction of said film, thereby forming a biaxially-textured film, wherein the deposition flux incident plane is arranged parallel to the direction along which the biaxially-textured film has a fast in-plane growth rate.
2) Prior Art
Second-generation superconducting tapes, such as those based on Y1Ba2Cu3O7-δ (YBCO) films, are being developed to carry large amounts of electrical current without electrical resistance. Such second-generation, high temperature superconductors (HTS) typically comprise biaxially-textured (narrow out-of-plane and in-plane grain orientation distributions) layers deposited on a metal substrate, such as a flexible metal tape. It is known that the biaxially-textured layer enables high current densities (Jc) in YBCO films, among others.
The effects of grain boundary characteristics on current transmission have been demonstrated for YBCO films (Dimos et al. (1988) Phys. Rev. Lett. 61:219; and Dimos et al. (1990) Phys. Rev. Lett. 41:4038). For clean, stoichiometric boundaries, critical current density appears to be determined primarily by grain boundary misorientation.
Several attempts have been made to grow sharply textured YBCO films having high critical current densities on flexible metal tapes. In one approach, a biaxially-textured layer was deposited using ion beam assisted deposition (IBAD) on a Ni-based alloy tape, such as Hastelloy® (S. R. Foyltn et al., IEEI Transactions on Applied Superconductivity 9 (1999) pp. 1519). The IBAD of a buffer layer of yttria-stabilized zirconia (YSZ) was the first demonstrated process to achieve biaxially-textured layers, and has produced several of the longest and best performing YBCO superconductors. It is generally accepted that texture development in IBAD-YSZ is based on a growth competition mechanism. As a result, one disadvantage of this method is that thick layers must be grown in order to achieve good in-plane texture. Typically, biaxially-textured layers that are more than about 1,000 nm thick achieve in-plane textures of less than 15° full-width-at-half-maximum (FWHM). This problem is further exacerbated by the very low deposition rate (about 0.1 nm per second) needed to grow high quality IBAD-YSZ. The combination of thick films and low deposition rates necessitates long deposition times (typically hours) to grow a biaxially-textured layer with a thickness greater than about 1,000 nm. Therefore, this process may not be suitable for rapid, large-scale industrial applications.
IBAD of magnesium oxide (MgO) has been used to achieve very good biaxial texture in films about 10 nm thick using a deposition rate of about 0.1 nm/second (J. R. Groves et al., Proc. 2001 Intl. Workshop on Superconductivity, Honolulu, Hi. (Jun. 24-27, 2001), p. 3). This IBAD-MgO process, as such, could be about 100 times faster than IBAD-YSZ. However, this IBAD-MgO method requires at least three additional layers in the buffer structure; the first is an amorphous seed layer, the second is a thick homo-epitaxially grown MgO layer, and the third is yet another layer for better lattice matching with YBCO. By requiring three additional layers, additional time and effort are needed to process the buffer structure in IBAD-MgO. Furthermore, the biaxial texture of MgO is very sensitive to the roughness of the underlying substrate, as well as other factors. Therefore, it may be difficult to achieve high yields in the manufacture of IBAD-MgO-based layers.
Inclined substrate deposition (ISD) without the assistance of ion beam bombardment has been shown to achieve high deposition rates (K. Hasegawa et al, Proc. of 16th ICEC/ICMC, Amsterdam: Elsevier Science (1997), p. 1077; and M. Bauer et al. IEEE Transactions on Applied Superconductivity 9 (1999) p. 1502). These high deposition rates can minimize the time needed for coating long wires. However, the quality of film produced by ISD is poor compared to the quality produced using IBAD, and the c-axis in these ISD layers is tilted off surface normal. This makes the critical current density (Jc) anisotropic, and the critical current decreases greatly along the tilt direction. The films deposited by this ISD method tend to have a rough surface with a pattern similar to “roofing tiles.”
In an additional approach, ion beam nanotexturing (ITEX) of YSZ has been shown to produce biaxially-textured YSZ in a matter of a few minutes (R. P. Reade et al., Applied Physics Letters, Vol. 80, No. 8 (2002) p. 1352). ITEX is similar to IBAD, except that in the ITEX method, an amorphous YSZ layer is first deposited, then an oblique ion (Ar+) beam at an angle of about 55° is used to bombard the amorphous film with O2 in a chamber. The result is a crystalline texture in the top surface of the amorphous layer. This method is very rapid, but results in a very poor in-plane texture of about 45°. An in-plane texture of about 15° or less is necessary in order to achieve good properties in the YBCO layer when deposited upon the biaxially-textured layer.
Fast ion beam assisted deposition of cerium oxide (CeO2) biaxially-textured layers (Fast IBAD) has been shown to achieve much higher deposition rates than IBAD-YSZ (X. Xiong et al., “Rapid Deposition of Biaxially-Textured CeO2 Buffer Layers on Polycrystalline Nickel Alloy for Superconducting Tapes by Ion Assisted Pulsed Laser Deposition”, Physica C, 336 (2000) 70). In fast IBAD, the deposition rate is similar to the ISD method, but fast IBAD results in a better film quality, and the c-axis (z-axis) is not tilted off normal in the fast IBAD conductors as it is in the ISD-based conductors. However, the Jc obtained by this fast IBAD method is not as good as that of IBAD-YSZ. The texture of the biaxially-textured layer, especially the texture of YBCO deposited via fast IBAD, requires further improvement.
Thus, there is a need in the art for novel and robust processes for depositing, at a very high deposition rate, a biaxially-textured film on a continuously moving metal tape. Such processes should increase the deposition rate by at least ten times the conventional deposition rate of IBAD-YSZ of about 0.1 nm/second, resulting in a deposition rate of about 1.0 nm/second or greater. Such processes should substantially reduce production times. Such processes should result in grain alignment on a large-scale basis. Such processes should be used to develop manufacturing facilities for producing kilometer lengths of HTS-coated conductors at price and performance levels needed for numerous applications. Such processes should result in a high population of low-angle grain boundaries.